1. Field of the Invention
This invention relates to methods for removing stringers appearing in plated through-holes in copper-containing multilayer substrates such as printed wiring boards comprising copper layers and layers of reinforced resin, such as polyimide on glass cloth or epoxy on glass cloth.
2. Description of Related Art
During the foil microetching that takes place immediately before electroless copper plating to provide good adhesion of copper, "stringers" are formed in copper foil containing multilayer substrates such as printed wiring boards. These stringers or tails are often formed from strands of copper, polymeric fiber, or both. These tails, which emanate from the rough brass side of copper foil in plated through-holes, act as nucleation sites for the electroless copper and can introduce stress raisers or planes of weakness in the plated layer. Such stringers can also lead to instability and cause barrel cracking (i.e., cracking in the walls of plated through holes) and electrical failures. Attempts to remove these stringers by contacting them with ammonium persulfate and other known etchants have failed.